Antenna arrays are increasingly used in electronic communications, including in the aerospace defense industry and the wireless telecommunications industry, for example. Antenna array test and calibration solutions are used to measure parameters of the antenna arrays (which may be referred to as characterizing the antenna arrays). Conventional solutions for test and calibration depend primarily on a vector network analyzer, which requires the device under test (DUT) including the antenna array module, or module under test (MUT), to have radio frequency (RF) connectors, such as coaxial connectors, in order to perform the test and calibration. However, with the evolution of wireless communication technologies, antenna arrays with direct connections to (i.e., integrated with) radio frequency (RF) transceivers of DUTs, and having no RF connectors, are becoming increasingly common. Overall performance of such a DUT presently must be tested “over-the-air,” since there is no place to connect a coaxial cable from the DUT and/or the antenna array to test equipment. In fact, due to antenna array integration, overall DUT performance must now be tested as a function of the antenna array configuration. When the antenna array is designed to produce signal beams, for instance, then the DUT performance must be characterized over a range of beam angles and/or widths.
Conventional solutions for over-the-air (OTA) testing are aimed primarily at single antenna measurements. However, with the advent of millimeter wave (mmW) bands and corresponding wireless communication standards, such as IEEE 802.11ad, as well as the advent of 5G networks, cost, size and speed become key attributes of test methodology. To characterize performance, various attributes of the DUT, such as radiation profile, effective isotropic radiated power, total radiated power, error-vector-magnitude (EVM) of the modulation, and adjacent channel leakage ratios (ACLRs), for example, are characterized as a function of beam angle. Currently, this involves a very time-consuming process. For example, characterizing just the radiation profiles of a DUT as a function of beam angle may take hours.
Antenna characterization processes typically take place either at an outdoor test range or in an anechoic chamber. Outdoor test ranges are used for antennas having a very long far-field (e.g., greater than 5 m), rendering use of an indoor test range or anechoic chamber impractical. Anechoic chambers are shielded, including walls covered in absorbing material that minimizes internal reflections, typically by several tens of decibels.
There are a number of basic conventional techniques for antenna characterization using an anechoic chamber, such as a simple-far-field measurement technique for an antenna having a far-field that occurs at a sufficiently short distance that it can be measured directly in a chamber of practical size, and a near-field measurement technique, according to which near-field measurements are mathematically transformed to the far-field. Another conventional technique uses a compact antenna test range (CATR), where an approximately uniform source (a single antenna) illuminates a curved mirror where the resulting reflection is collimated. In this way, the DUT with a long far-field distance may be positioned in the collimated beam, and the DUT antenna's radiation pattern may be determined as the received power changes as a function of rotation angle (elevation and azimuth) of the DUT. The collimated reflection from the curved mirror allows the DUT to be characterized in the far-field in a more compact chamber than would otherwise be possible without the curved mirror.
As mentioned above, mmWave bands are being used in 5G networks in order to obtain wide enough bandwidth to enable high throughput. The high frequency bands tend to have high path loss, which generally requires use of antenna arrays to achieve higher antenna gain to offset the effects of the high path loss. Phased arrays (referred to herein as “phased antenna arrays” or simply “antenna arrays”) are commonly used in devices for these high frequency bands, including both user equipment (UE) and base stations, such as eNodeBs/gNodeBs. In order to make the beamforming (BF) process efficient for antenna arrays, hybrid beamforming may be implemented. According to hybrid beamforming, techniques used in analog as well as digital beamforming are combined.
5G systems at higher frequencies (mmWave) apply integrated analog beamforming with a fast dynamic beam switching procedure, which may be tested OTA when RF-antenna connectors at each antenna element are not available. Both base station and user equipment (UE) may apply analog beamforming Each antenna port is connected to multiple antenna elements, and analog beamforming is applied for testing at each antenna port using a fixed set of element weighting coefficients, such that different beams may be chosen per orthogonal frequency division multiplexing (OFDM) symbol basis.
The effect of the analog beamforming (or other time-variant or time-invariant beamforming) may be simulated in conductive emulation by embedding correct antenna array radiation pattern (beam pattern) into the channel model at each time instant, as described, for example, by Kyosti et al., U.S. Pat. No. 9,407,381 (Aug. 2, 2016), which is hereby incorporated by reference in its entirety. According to the 3GPP standard, a 5G base station may have at maximum of 64 different fixed analog beam directions per antenna port for initial access beam sweeping. Beam sweeping may be performed by periodically transmitting synchronization and reference signals by different analog beams on a broadcast channel (BCH) for initial access and beam management purposes. During the beam sweeping on the BCH, the beams are changed in blocks of four OFDM symbols, referred to as synchronization signal (SS) blocks. Each beam may be defined by a fixed set of antenna element weighting coefficients.
During OTA testing, the DUT is placed into an anechoic chamber and the other link end (e.g., a communication tester or an UE emulator) may be connected to a channel emulator through cables or OTA. The beamforming procedure may be included in the testing using actual DUT antenna array and beamforming procedures if all clusters of a channel model (“isotropic channel”) are included in the OTA channel model. The beamforming antenna array with directive narrow beams filters out effectively weak multipath clusters of the channel model. This means that the number of significant clusters is reduced compared to non-directive antenna systems, for example. The number of possible beams at the transmitter and/or the receiver of the DUT may be large, but the number of significant beam pairs may be much lower and many weak beams may be neglected in testing.
However, many aspects in the testing of 5G mmWave devices are related to testing of operation and performance of beam selection and tracking procedures. Thus, multiple beams are required for adequate testing. For example, beam management testing may involve beam switching, which requires at least two beams. Implementation of an OTA isotropic channel model requires inclusion of all clusters of a channel model into an OTA model. This means that the required number of probes (and channels in a channel emulator) in the anechoic chamber depends on the number of spatially separable clusters and rays in the channel model. Generally, clusters are components of a geometry-based stochastic channel model (GSCM) that represent propagation pathways of a multipath channel model. Each cluster consists of a number of rays (propagation paths). In a GSCM model, each cluster is defined to have certain angle of arrival, angle of departure, delay and power, as well as arrival and departure angle spreads. These parameters may be referred to as small scale channel model parameters. The rays of a GSCM model are generated for each cluster according to cluster angles and ray angle offsets determined according to cluster angle spreads. In accordance with current 3GPP model specifications, there are 20 rays per cluster. A fading model is implemented by a so-called sum-of-sinusoids method, in which each ray represents a sinusoid with an arrival angle specific Doppler phasor and the rays are summed to obtain a fading “tap” for each cluster.
A narrow beam device may pick clusters and rays of the channel model with high resolution, i.e., the device accurately samples the power angular spectrum (PAS) of the channel model. Therefore, the number of required probes in the isotropic channel model is very high for narrow beam devices.